Cyclic-voltammetry study of bismuth telluride electrodeposition in polycarbonate etched ion-track membranes

Bismuth telluride compounds are well-known to be among the most efficient thermoelectric materials at room temperature. Currently, some efforts are made to synthesize nanowires, since theoretical predictions have shown that their transport properties can be favourably modified compared with bulk materials. The feasibility of the electrochemical synthesis of bismuth telluride nanowires in porous membranes has been already demonstrated, but literature data report opposite effects of the nanostructures on the Seebeck coefficients, exhibiting both lower and higher values than those of bulk references [1]. This disparity among data can be due to the chemical composition of the nanowires. Indeed, the thermoelectric properties of chalcogenide compounds are directly linked to their stoichiometry. The aim of this work was to define a specific protocol of electrodeposition, leading to Bi2Te3 nanowires with controlled composition and with high filling ratio. Thus we have analyzed the electrochemical deposition of bismuth telluride in etched ion-track polycarbonate membranes, which acts as an array of embedded ultra microelectrodes. Polycarbonate foils of thickness 30 μm were irradiated with gold ions (E = 11.1 MeV/u) at the UNILAC accelerator. A fluence of 10 ions/cm was applied which is low enough to avoid pore overlapping in the resulting templates. After UV sensitizing, the pore diameters were adjusted to 60, 90, and 120 nm by chemical track etching in 6N caustic soda at 50°C. All electrochemical experiments were carried out at room temperature with a three-electrode setup that consisted of the track-etched template with a 160-nm thick sputtered platinum layer on one side acting as the working electrode, a saturated Ag/AgCl reference electrode, and a platinum disk counter-electrode. All electrolytes contained 10 mM Te, 1N HNO3, 50% v/v dimethyl sulfoxide (DMSO), and different concentrations of Bi. DMSO as a wetting agent. Further experimental details can be found in reference [2]. In a first set of experiments, bismuth telluride nanowires were grown in acidic media under potentiostatic conditions at U = -100 mV with electrolytes containing Bi and HTeO2 + at equal concentrations. This value of -100 mV was chosen based on previous results for stoichiometric films. The filling ratio of the pores was increased from 51 to 80% by adding DMSO to the electrolyte. The global composition of the samples displayed a large excess of Te (80 at.%) versus Bi (20 at.%), with or without DMSO in the electrolyte [2]. Figure 1: Cyclic voltammograms at 5mV.s within 120 nm diameter pores membrane with and without DMSO, |Bi|=|Te|=10M